Polymers such as polypropylene may be prepared into articles having reduced haze by adding clarifying agents into the polymer. Solid plastic articles such as food storage containers, household plastic products, and the like are commonly rendered clear by the addition of clarifying agents to the polymers. Clarifying agents are commonly added to polymers as part of an “additive package” pre-blend which may contain in addition to clarifying agents one or more of the following: antioxidants, acid scavengers, slip agents, light stabilizers, optical brighteners and UV light absorbers.
Referring now to FIG. 1, in the conventional use of clarifying agents, various customary plastic additives may be combined with a clarifying agent. Then, the mixture may be low or high intensity mixed. A polymer is added, and then the resulting batch may be mixed, followed by heat compounding to form a polymer. A forming process such as injection molding, thermoforming, injection stretch blow molding (“ISBM”), or extrusion blow molding (“EBM”) may be accomplished in the manufacture of articles.
Sometimes a coloring agent (which may be in the form of a masterbatch pre-mixed with polymer) is added during or just prior to the forming process, as in FIG. 1. This procedure is one manner in which plastics are colored in the industry. A relatively high colorant loading of 500-1000 parts per million (ppm) may be mixed and adequately dispersed into a plastic in this manner. For loadings in the range of about 500-1000 ppm, this procedure is satisfactory. A conventional process is suitable for applying a high degree of color to make brightly colored plastic articles for everyday use.
It is difficult to adequately disperse an additive into a plastic or polymer at extremely low additive concentration levels. For example, dispersing an additive into a polymer at very low loading levels of additive usually requires several steps. For example, an additive may be added to a polymer in a “let-down” to bring the additive into a range in the range of roughly 10,000 of parts per million (ppm). All ratios are herein by weight unless otherwise specified. Then, another “let-down” step may be needed to bring an additive into a range of a few hundred ppm. Sometimes, a third let-down may be needed to bring an additive to a range of less than about 10 ppm, or less than 5 ppm. Thus, applying additives in a range of only a few ppm must be done in discrete steps, and usually cannot easily be accomplished conventionally using typical manufacturing equipment in only one step. It is difficult to achieve adequate and homogenous dispersion of the additive in the polymer. Dispersion is very difficult to achieve when using very low loadings of an additive. Achieving a very high degree of dispersion at low loadings is difficult and time consuming in most polymer manufacturing applications.
There is a continual search in the industry for additives that will improve the visual appearance or clarity of polymeric articles. In the case of substantially uncolored (“clear”) plastic articles, it is common to conduct haze testing to determine the level of haze exhibited by a clarified plastic article or plaque. In general, the visual appearance of a plastic article is commercially significant in the plastics manufacturing industry.
Plastic articles such as storage containers typically are stacked for display in retail stores. When four or five uncolored plastic articles (such as food storage containers) are stacked on a retail shelf, the collective haze of the stack is “multiplied” based upon the number of articles in the stack. Thus, an article (product) that may otherwise appear to be clear when viewed alone may not appear to be nearly so clear when four or five or more like articles are stacked and nested into each other on a store shelf.
Consumers find clear plastic to be highly desirable in many product applications. For example, in the case of storage containers, consumers may desire to see the contents of the container. Containers that are clear may command a premium in the market for storage containers. Additives that improve the visual appearance of plastic are very desirable. Plastic additives that improve the visual appearance of plastic articles when such articles are stacked or nested are very desirable. This invention is addressed to make such improvements.